1. Field of the Invention
The present invention relates to the field of analog signals, and more specifically, to the restoration of DC levels in analog signals prior to conversion to digital form.
2. Prior Art
DC restoration is the process by which the quiescent value of a waveform is established with respect to a reference value. The deviation from the reference value is an error. DC restoration is accomplished by a machine called a servomechanism, specifically a regulator, and can be analyzed as such. For video signals, all presently available commercial implementations only regulate a video level to an analog reference.
Important applications exist, including but not limited to video, where the input and output variables are in different domains. The input is an analog value and the output is a digital value, with the error specified in terms of the LSB at a given point on the waveform.
An analog video signal is comprised of a camera signal (image information) periodically interrupted by blanking intervals. Each horizontal blanking interval consists of a front porch signal level, followed by a horizontal sync signal, followed by a back porch level and then by the camera signal again. The front and back porch levels are the same, corresponding to the Black level for the camera signal, with the horizontal sync signal being in the opposite sense to the camera signal, sometimes referred to as Blacker than Black.
The front porch period is nominally only one fourth as long as the horizontal sync period, though the back porch is nominally three fourths as long as the horizontal sync period. For definiteness in distinguishing the front and back porches from the horizontal sync signal, the Black level is intentionally set somewhat “above” the level that will be interpreted as the horizontal sync level. An example is the ITU-R BT.601 Specification of Studio Encoding Parameters for TV, in which the digital level of Black is given as 16 and 64 for 8 or 10 bit quantizations, respectively. It is important to accurately maintain the Black level not only to avoid falsely interpreting the same as a horizontal sync signal, but also to provide an accurate reference for Black in the camera signal. The back porch, being longer in time and defined as Black, becomes a convenient reference for sensing and correcting the Black level.
An example of a design used for such applications is shown in FIG. 1. The input (Analog Video In) is gained up by the operational amplifier (op-amp) A1, whose output is compared during a sample interval with an analog reference Restore Voltage by op-amp A2. At the end of the sample interval, the output of the comparison is held in the sample and hold circuit S/H until the end of the next sample period. Resistor R7 and capacitor C3 provide a filtering or averaging function during the sample interval.
The above approach has several problems.
1. Although a closed loop servomechanism, it regulates the output to an analog voltage, not the quantized digital value of Black specified.
2. The loop is closed around the input op-amp A1, not the differentially biased input to the analog to digital converter (ADC), leaving out the error in the anti-aliasing filter and Vcom (common mode voltage). These can be added in existing designs, but add several op-amps to the circuit.
3. It has a finite hold time determined by the S/H leakage, causing “droop” during a horizontal line.
Solutions to these problems have appeared as various forms of “Infinite S/H” where the reference is replaced by a D/A decoding of the digital value of Black level, as shown in FIG. 2. The output of op-amp A1 is still open loop, passing through the Filter, a single ended to differential conversion (SE To Diff) and then to the A/D converter. This removes the errors due to the finite S/H, but doesn't form a closed loop to reduce the variations with time and temperature. The fact that the reference is determined in the same domain is often enough to get the error within a few LSBs. This is the result of weak correlation due to common supply and reference voltages.
While video has been used as an exemplary application, the present invention is applicable to other applications. By way of but one example, the invention is applicable to correlated double sampling, such as with contact image sensors, where the desired output is the difference between the current output of each pixel sensor when exposed to the image and the current output when that pixel is not illuminated, referred to as the “Dark Current Offset”.